Method for producing ammonium polyphosphate

ABSTRACT

A method of continuously producing ammonium polyphosphate by the reaction of urea and polyphosphoric acid, over a wide range of molecular weights, in a pre-mix slurry, under controlled temperature of about 315°+/-15° C. The slurry is reacted on a hot, continuously moving surface and forms ammonium polyphosphate in an ammonia atmosphere. Preferably, the pre-mixed slurry is added to a screw-type extruder, where the material comes in contact with the ammonia along the preheated moving surfaces of the extruder. The ammonium polyphosphate is scraped from the extrusion screw into a container with minimum handling. The material can be ground to a finer consistency if desired.

BACKGROUND OF THE INVENTION

This invention relates to the production of ammonium polyphosphate. Ithas particular, but not exclusive, application to producing the type ofwater-insoluble ammonium polyphosphate useful in fire- andheat-resisting materials.

The term ammonium polyphosphate is used to describe a family ofcompounds, ranging from water soluble, low molecular weight materialsutilized as fertilizers, to highly water insoluble long chain moleculesused as starting materials in various manufacturing processes, asfire-fighting materials, and as additives to coatings to give thecoatings fire-resistant and heat-resistant properties.

Numerous methods have been proposed and used for producing ammoniumpolyphosphate. Liquid fertilizer compositions have been formed fromdirect ammoniation of superphosphoric acid (a mixture of orthophosphoricacid and polyphosphoric acid) with anhydrous ammonia. For example, inHignett et al, U.S. Pat. No. 3,171,733, superphosphoric acid (76% P₂ O₅)and anhydrous ammonia are reacted at elevated temperatures andcontrolled pressure. The process requires continuous agitation and iscompleted in a time period of one hour or more. The product is a watersoluble, linear, low molecular weight material.

In many applications, it is highly desirable to utilize an ammoniumpolyphosphate having a water solubility of less than about two percent.Materials have been known for about twenty years, having a solubility ofabout 1.5 percent and are known as "substantially water-insoluble"ammonium polyphosphates. They are believed to be crosslinked, highmolecular weight, crystalline materials. Commercial substantiallywater-insoluble ammonium polyphosphate is sold by Monsanto Company underthe designation "PhosChek P-30." It may be noted that the commercialmaterial gives two distinct and almost equal thermochemicaldecompositions, and is believed to be formed by reaction of urea withorthophosphoric acid. It is therefore believed that the material mayactually be a chain incorporating both ammonium polyphosphate and eithercyanuric acid or cyamelide in nearly equal proportions, although thissurmise has not yet been conclusively demonstrated. The term"substantially water-insoluble ammonium polyphosphate" as used hereinincludes this material. The substantially water-insoluble ammoniumpolyphosphates are high molecular weight materials, characterized byP-O-P-bonding.

The known processes for forming crystalline, substantiallywater-insoluble ammonium polyphosphates are generally characterized byforming the polyphosphate backbone in situ. For example, Shen et al,U.S. Pat. No. 3,397,035, produces the long-chain material by mixingabout a 1:1 ratio of substantially water-insoluble crystalline ammoniumpolyphosphate with a reaction mixture of inorganic phosphate and aninorganic source of ammonia, and heating the mixture. These processesare slow, difficult to control, give limited yield, and require theexpenditure of large amounts of energy.

SUMMARY OF THE INVENTION

One of the objects of this invention is to provide a method forproducing substantially water-insoluble ammonium polyphosphate in largequantities, in a high yield, and with low energy expenditures.

Another object is to provide such a method which is controllable toprovide product of different molecular weights and compositions.

Another object is to provide such a method which produces a product inwhich substantially all the polymeric linkages are between phosphorusand oxygen atoms (P-O-P bonds).

Another object is to provide such a method which may be carried outcontinuously, using conventional equipment.

Other objects of this invention will be apparent to those skilled in theart in light of the following description and accompanying drawings.

In accordance with one aspect of this invention, generally stated, aprocess is provided for the manufacture of ammonium polyphosphate whichincludes a first step of forming a mixture consisting essentially of onepart of a polyphosphoric acid having an equivalent phosphoric acidcontent of at least about 110% (80% P₂ O₅) and from about 0.5 to 3.0parts of urea by weight, and a second step of heating the mixture to atemperature of at least about 150° C. to ammoniate substantially allphosphorus atoms in the polyphosphoric acid.

In the method of the invention, the decomposition of urea is believed tobe initiated by the transfer of a proton from the polyphosphoric acid tothe oxygen atom of the urea, and not by the spontaneous thermaldecomposition of the urea. Therefore, the initial ammoniation rate ofthe polyphosphoric acid varies inversely with the initial molecularweight of the polyphosphoric acid. A highly suitable reaction rate isachieved using the preferred polyphosphoric acid having an equivalentphosphoric acid content in the range of 115% to 117%. This material isbelieved to consist primarily of trimeric phosphoric acid.

In the process of the invention, an exothermic reaction takes placebetween the polyphosphoric acid and urea and forms, almostinstantaneously at a temperature of from 150° to 200° C., an averagepenta ammonium trimer, in the form of a finely divided white crystallinesolid. This material has a solubility of about 8%. The material isuseful in this form, or it may be converted into a less soluble ammoniumpolyphosphate in accordance with a further aspect of the invention. Thereaction also produces cyanic acid. The fate of the cyanic acid is notyet known, but it may either be lost as a vapor or it may mix in thesubstantially water-insoluble ammonium polyphosphate either mechanicallyor by polymerization with it as cyanuric acid or as cyamelide.

In accordance with another aspect of the invention, a process isprovided for the continuous manufacture of substantially water-insolubleammonium polyphosphate, which includes a first step of mixing one partof a polyphosphoric acid and an excess of urea, and a second step ofheating the mixture to a temperature of at least 300° C. to form asubstantially water-insoluble ammonium polyphosphate.

Preferably, the reactants consist essentially of the polyphosphoric acidand urea, and the reaction is carried out under an ammonia atmosphere.The ammonia atmosphere prevents decomposition of the ammoniumpolyphosphate during polymerization, but it is preferably at or slightlyabove atmospheric pressure, to maintain the material in a workable form.

Preferably, the second step of the method is carried out in a heatedextrusion device providing continuous heating of an extruder screw whichcontinuously forces the reaction mixture through a reaction barrel. Thereaction is carried out by continuously introducing the reactants intothe barrel, and using both applied heat and the heat generated by theammoniation step of the reaction to raise the temperature to about 305°C. The closed barrel contains the ammonia given off during the reactionand maintains the self-generated ammonia atmosphere. Venting is utilizedas necessary to maintain a proper pressure within the reactor barrel.The substantially water-insoluble ammonium polyphosphate emerges fromthe extruder as a mass which is preferably ground to the desiredfineness before use.

Preferably, the ammonium polyphosphate has an equivalent phosphoric acidcontent of at least about 110% (80% P₂ O₅), most preferably from about115% to about 120%.

Preferably, the urea is present in a quantity of from 0.5 to 3.0, mostpreferably about 0.8 to 1.2 parts by weight of the polyphosphoric acid.

Preferably, the reaction is carried out at a temperature of 315°+/-15°C. for a period of from 30 minutes to 180 minutes.

Preferably, the reaction mixture is heated for a period sufficient toprovide an ammonium polyphosphate which is less than 1% soluble. It hasbeen found that the present process permits the continuous production ofammonium polyphosphate having a solubility of less than 1%, down toabout 0.1%.

Other aspects of the invention will be better understood in light of thefollowing description of the preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred processes of the present invention are set out in thefollowing examples.

EXAMPLE 1 Production of Partially Soluble Ammonium Polyphosphate

Into a reactor are added one part of polyphosphoric acid having anequivalent phosphoric acid content of 115% (primarily trimericphosphoric acid) and one part by weight of finely divided urea. Themixture is heated gently, and an exothermic reaction occurs. At about100° C. the mixture turns to a milky consistency, and at 140° C. itturns clear. At about 154° C. a highly exothermic reaction occurs,increasing the temperature of the reaction mixture to about 200° C.within less than five minutes. A finely divided ammonium polyphosphateis formed. The material has a solubility of about 8% (equilibrium).

EXAMPLE 2 Batch Production of Substantially Water-Insoluble AmmoniumPolyphosphate

The finely divided ammonium polyphosphate produced in accordance withExample 1 is placed in a closed reactor under an ammonia atmosphere andheated with constant mixing. When the temperature of the mixture isincreased to 300° C. for a period of fifty minutes, the resultingmaterial is a substantially water-insoluble ammonium polyphosphatehaving a solubility of less than 1.5%. The material appears to besimilar to commercially available substantially water-insoluble ammoniumpolyphosphate in pH, solubility, thermogravimetric analysis, infraredspectrum, stability in water-based systems, and heat resistingproperties. Yield is in excess of 70%.

Heating the material obtained from the process of Example 1 at 300° fora longer period further decreases the solubility of the substantiallywater-insoluble ammonium polyphosphate. Heating it at 310° providessubstantially the same material in substantially the same time,independent of the partial pressure of ammonia and independent of thesize of the batch. At 290° , however, only very small batches ofmaterial may be successfully polymerized to substantiallywater-insoluble ammonium polyphosphate.

EXAMPLE 3 Continuous Production of Substantially Water-InsolubleAmmonium Polyphosphate

This Example utilizes a screw-type extruder having an internally heatedscrew. The barrel of the extruder has a diameter of one foot and alength of twenty feet. The volume between the extruder wall and thescrew forms a closed reaction vessel. The screw surface is heated to aconstant temperature of 310° C. The extrusion rate is chosen to providea contact time of less than one hour. Into the extruder is continuouslyintroduced a slurry consisting of one part of polyphosphoric acid havingan equivalent phosphoric acid content of 115% (primarily trimericphosphoric acid) and one part by weight of finely divided urea. Theatmosphere within the extruder is quickly filled with outgassed ammoniafrom the urea. The resulting substantially water-insoluble ammoniumpolyphosphate may be ground to a fine powder. It has a solubility ofless than 1.5% and exhibits properties substantially similar to thesubstantially water-insoluble ammonium polyphosphate produced inaccordance with Example 2.

Numerous variations, within the scope of the appended claims, will beapparent to those skilled in the art in light of the foregoingdescription.

We claim:
 1. A process for the manufacture of substantiallywater-insoluble ammonium polyphosphate, comprising a step of mixing onepart of a polyphosphoric acid having an equivalent phosphoric acidcontent of at least about 110% and an excess of urea, to form apartially soluble ammonium polyphosphate having a solubility of about 8%in water, and a second step of heating the partially soluble ammoniumpolyphosphate under an ammonia atmosphere to a temperature of at least300° C. for a period sufficient to form a substantially water-insolubleammonium polyphosphate.
 2. The process of claim 1 wherein said secondstep is carried out with constant stirring for a period of at leastabout thirty minutes.
 3. The process of claim 2 wherein said partiallysoluble ammonium polyphosphate is heated to a temperature of 315° +/-15°C.
 4. The process of claim 1 wherein the reactants consist essentiallyof said polyphosphoric acid and urea.
 5. The process of claim 4 whereinsaid partially soluble ammonium polyphosphate is heated to a temperatureof 315° +/-15° C.
 6. The process of claim 1 wherein said second step iscarried out in a heated extrusion device providing continuous heating ofan extruder screw which continuously forces the reaction mixture througha reaction barrel.
 7. The process of claim 6, wherein said second stepis carried out in a heated extrusion device having an extruder screwwhich continuously forces the reaction mixture through a reactionbarrel.
 8. The process of claim 7 wherein a mixture of saidpolyphosphoric acid and said urea is continuously fed into the barrel ofsaid extruder.
 9. The process of claim 1 wherein said ammoniumpolyphosphate has an equivalent phosphoric acid content of from about115% to about 120%.
 10. The process of claim 1 wherein said urea ismixed in a quantity of from about 0.5 to 3.0 parts by weight of thepolyphosphoric acid.
 11. The process of claim 1 wherein said urea ismixed in a quantity of from about 0.8 to 1.2 parts by weight of thepolyphosphoric acid, and wherein the reaction is carried out at atemperature of 305°+/-5° C. for a period of from 30 minutes to 180minutes.
 12. The process of claim 1 wherein the reaction mixture isheated for a period sufficient to provide an ammonium polyphosphatewhich is less than 1% soluble.